290 ANATOMY OF VERTEBRATES. 



the difference in the shape, size, and structure of their prosence- 

 pliala is hardly less than that between the Shark and Pike. The 

 combative Stickleback has longer and narrower prosencephala 

 than the cowardly Gudgeon. The nidificative and philo-pro- 

 genitive Callichthys has neither the antero-lateral nor the posterior 

 regions of the cerebrum more developed than in bony fishes 

 generally. 



53. Myelencephalon of Reptiles. The perennibranchiate Bn- 

 trachia lead sluggish lives in swamps and pools ; their senses are 

 as little developed as those of the Lepidosiren, and their mus- 

 cular movements are perhaps even more restricted : hence, if the 

 cerebral lobes seem to preponderate, in proportion to other parts 

 of the brain, over the prosencephalon of Osseous Fishes, it is 

 rather by contrast with the rudimentary condition of the mes- and 

 ep-encephala than in the relative size of the prosencephalon to the 

 entire body. 



In a Newt, weighing 39 grains, the brain weighs one-seventh 

 of a grain : and in the large Sirens, Amphiumes, and Menopomes, 

 the proportion of the brain to the body is less than in the 

 Newts. 



The medulla oblongata slightly expands; the post-pyramidal 

 and restiform tracts diverge and expose a long and simple ' fourth 

 ventricle,' with a median fissure : the convergence and confluence 

 of the borders at the fore part of the ( calamus ' offer a feeble 

 rudiment of cerebellum. The optic lobe in the Axolotl is a long 

 elliptical body, two-thirds the breadth of the epencephalon. A 

 slight swelling below gives off the small optic nerves, and is 

 produced anteriorly into a vascular f hypophysis ' : a larger pineal 

 body extends from before the optic lobe upon the posterior inter- 

 space of the cerebral lobes, completing the mesencephalon, which 

 is the smallest of the primary divisions of the brain. The 

 cerebral hemispheres, twice the length and breadth of the optic 

 lobe, are smooth and hollow, like those of Lepidosiren. The 

 olfactory lobes are pyriform, with the base sessile on the fore 

 and outer part of the hemispheres ; the nerve is shorter than in 

 Lepidosiren. The cerebral ventricles are continued into the 

 olfactory lobes. 



The small and simple brain may be wholly removed from a 

 torpid batrachian in the winter season, the medulla oblongata 

 included, by section of the myelon in front of the roots of the 

 second pair of cervical nerves ; and, nevertheless the animal 

 survives many weeks, preserving the reflex actions of the myelon 

 and nerves, the contractility of the muscular fibre, and the 



